As an RPG-fan, should I buy the PSP or DS

[TJC]

I'd say, go for both.
You can find DS phat or the old DS lite dirtcheap and they have a thriving homebrew community (colors is awesome fun to draw with, moonshell makes a great audio and video playing device out of your DS) and it has RPGs outside the box:
Mario & Luigi: Partners in Time and Bowser's Inside Story are hilarious, decently written and beautifully drawn and animated.
The World Ends With You is unique in its setting and the use of the DS features is awesome.
Radiant Historia is just plain awesome and another piece of evidence that Atlus >> Squeenix.
and if you get it, check the highly underrated Valkyre Profile: Covenant of the Plume.

On the PSP side, you get horse power and a lot of more choice for the classical J-RPGs but I can't say I've dabbled too much in that area because old-school RPGs just can't capture me the way the did before :/
but, nonetheless, the little thing isn't all that bad and you can get a used one just as cheaply.

 

[LordRoyal]

For four, it has the PSN, which holds a good number of PS1 games that all work on it. I don't know how well you can control all of them, considering that some of the control schemes have to be jerry-rigged to work on the PSP's limited button choice, but you can change what buttons do what if need be.

The PSP allows you to alter the game's control scheme, by default the L2 and R2 buttons are mapped to moving the analogue stick left and right respectively. You can also choose to use the analogue stick as the movement cursor, and choose to remap the entire gamepad to your hearts delight if you so require.

 

[roneel]

PSP is my preferred dose for rpg goodness

Valkyria Chronicles 2 is great
Final Fantasy (various titles)
and if you have a ps3 you can link your games from it onto the psp, like crash team racing lol

 

[AbstractStream]

As an owner of both systems, I'd say I have a lot more RPG games for my PSP than my DS.
I'd say go for the PSP. IF you have the money, definitely go for both. It's worth it.

 

[Thaius]

I own both, and the DS is superior on all fronts. And in terms specifically of RPGs, it has The World Ends With You, so it automatically wins right there, but it also has RPGs like the Final Fantasy IV remake, The Mario and Luigi games, and the more visual novel-esque classics like Phoenix Wright and Hotel Dusk. It gets my vote without question.

Also, both have some good multiplatform RPGs like Puzzle Quest, which you absolutely should get.

 

[ModReap]

As an RPG-fan, you should definitely go for the DS.